The invention concerns an X-ray optical system with an X-ray source and a first graded multi-layer mirror, wherein the extension Qx of the X-ray source in an x direction perpendicular to the connecting line in the z direction between X-ray source and a first graded multi-layer mirror is larger than the region of acceptance of the mirror at a focus of the mirror in the x direction.
A system of this type is known e.g. from “X-Ray Microscopy”, V. E. Cosslett et al., Cambridge at the University Press, 1960 which describes the principal operating mode of an arrangement of this type.
A concave focusing X-ray mirror can have a cylindrical, elliptical, or parabolic surface of curvature. When parabolic mirrors are used, the impinging X-radiation can, in particular, be rendered parallel.
The use of multi-layer mirrors in connection with a Kirkpatrick-Baez arrangement is described in an article by J. Underwood in the journal, Applied Optics, Vol. 25, No. 11 (1986).
As background discussion of the magnitudes of the quantities of interest, it is noted that the angle of acceptance of typical multi-layer mirrors is in the region of 1 mrad and typical foci in the region of several centimeters. The electron focus of the X-ray source varies in a linear range of 10 μm to a few millimeters. The acceptance of one mirror has a minimum linear size in the region of a few 10 μm and is typically striped. However, typical X-ray samples have linear extensions in the range of 100 μm up to a few millimeters and typically several tenths of a millimeter.
One main problem with X-ray optical systems of this type having extended X-ray sources, is that only X-ray radiation from a relatively small surface region of the electron focus satisfies the Bragg condition for diffraction on the graded multi-layer mirror (=Göbel mirror). For this reason, only a small part of the useful emitted radiation is guided from the X-ray source via the X-ray mirror in a predetermined desired direction. The entire surface of the X-ray source emits disturbing radiation (with a “wrong” wavelength, in particular Kβ) which can pass, via the X-ray mirror, through the entire apparatus to finally gain entrance to the X-ray detector.
In view of the above, it is the object of the invention to present an X-ray optical system with the above-mentioned features which facilitates reduction of the disturbing radiation on the sample with unchanged useful X-radiation source power and with a minimum of technically straightforward modifications.